1. Field of the Invention
This invention relates generally to an optical fiber link including a Raman fiber amplifier that employs a depolarized pump source and, more particularly, to an optical communications system including a fiber transmission link having a Raman amplifier that employs a depolarized pump source.
2. Discussion of the Related Art
Optical communications systems employ optical transmission fibers to transmit optical signals carrying information over great distances. An optical fiber is an optical waveguide including a core having one index of refraction surrounded by a cladding having another, lower, index of refraction so that light signals propagating down the core at a certain angle of incidence are trapped therein. Typical optical fibers are made of high purity silica including certain dopant atoms that control the index of refraction of the core and cladding.
The optical signals are separated into optical packets to distinguish groups of information. Different techniques are known in the art to identify the optical packets transmitted through an optical fiber. These techniques include time-division multiplexing (TDM) and wavelength-division multiplexing (WDM). In TDM, different slots of time are allocated for the various packets of information. In WDM, different wavelengths of light are allocated for different data channels carrying the optical packets. More particularly, sub-bands within a certain bandwidth of light are separated by predetermined wavelengths to identify the various data channels.
When optical signals are transmitted over great distances through optical fibers, attenuation within the fibers reduces the optical signal strength. Therefore, detection of the optical signals over background noise becomes more difficult at the receiver. In order to overcome this problem, optical fiber amplifiers are positioned at predetermined intervals along the fiber, for example, every 80-100 km, to provide optical signal gain. Various types of fiber amplifiers are known that provide an amplified replica of the optical signal, and provide amplification for the various modulation schemes and bit-rates that are used.
A popular optical fiber amplifier for this purpose is an erbium doped fiber amplifier (EDFA) that provides optical amplification over the desired transmission wavelengths. EDFAs are common because erbium atoms provide light amplification over a relatively broad wavelength range, for example, 1525-1610 nm. The erbium-doped fiber within the EDFA is pumped by a pump laser at a certain excitation frequency, such as 980 nm or 1480 nm. These wavelengths are within the absorption bands of the erbium, and results in the generation of optical gain in the wavelength range of 1550 nm. Thus, for an optical amplifier having a signal with a wavelength of 1550 nm propagating through the erbium-doped fiber, the signal is amplified by the stimulated emission of 1550 nm energy when the fiber is pumped by a 980 nm pump source. The pump light is absorbed by the erbium atoms that cause electrons in the atoms to be elevated to higher states. When a photon in the optical signal being transmitted hits an excited erbium atom, a photon of the same wavelength and at the same phase is emitted from an elevated electron, which causes the electron to decay to a lower state to again be excited to a higher state by the pump photons. The optical signal is amplified by the generation of additional photons in this manner.
Another type of fiber amplifier sometimes employed in a fiber communications link is a Raman amplifier. A Raman amplifier provides amplification within the fiber itself by launching pump light into the fiber from a pump source. The pump light provides optical signal gain by stimulated Raman scattering along the length of the fiber. Semiconductor lasers are generally used in the pump source to generate the pump light, and a wavelength division multiplexer (WDM) is used to couple the pump light into the fiber. Typically, the wavelength of the pump light is about 100 nm less than the wavelength of the signal light to provide the amplification. For example, to amplify signal light in the C and L bands (1520-1600 nm), lasers generating pump light in the 1420-1500 nm wavelengths are used.
The pump light can be launched in either the co-propagating or counter-propagating direction relative to the propagation direction of the optical signal. However, counter-propagating pump light typically has advantages over co-propagating pump light. Most optical communications systems employing Raman amplification take advantage of the counter-propagating pump configuration, where the pump light propagates in the opposite direction to the signal light. Counter-propagating the pump light has the advantage of vastly reducing the amount of pump noise transferred onto the signal channels, as well as minimizing the problem of pump-mediated cross-talk. As reach and information capacity of transmission systems are pushed into even higher limits, the desire to utilize both co-propagating and counter-propagating Raman pump configurations is increasing. Co-propagating Raman pumping gives system performance benefits because the signal powers are maintained at a more uniform power level to route each span of the system.
A Raman amplifier is more desirable than an EDFA in some optical amplification applications because it is able to provide amplification along a long span of the fiber as opposed to the EDFA that only provides amplification in the EDFA fiber. Because the Raman amplifier provides amplification along a long length of the fiber span, the signal strength of the optical signal does not fall to as low of a level as in those systems that employ only EDFAs. Thus, the noise figure of the Raman amplifier is generally very low. Also, because the EDFA is spliced into the fiber as a separate component, insertion losses are typically higher with an EDFA than with a Raman amplifier.
Because lasers are used to generate both the signal light and the pump light in a Raman amplifier, both the signal light and the pump light will be circularly, elliptically or linearly polarized. The relative polarization of the pump light and the signal light affects the amount of gain the pump light provides, and is known as polarization dependent gain (PDG). Particularly, if the signal light and the pump light are polarized in the same direction, then the amplifier provides the most gain. If the signal light and the pump light are polarized 90xc2x0 relative to each other, then there is virtually no gain. Relative polarizations between 0xc2x0 and 90xc2x0 provide different levels of gain depending on the angle. It has been found that a co-propagating Raman pump generally has a higher level of PDG compared to a counter-propagating pump at the same level of Raman gain and pump degree of polarization (DOP).
As the pump light and the signal light propagate through the fiber, their polarization will change as a result of various factors, including temperature, pressure, strain, etc., on the fiber. Further, each wavelength band for a particular channel in a WDM system may have a different polarization relative to the other bands. Therefore, the relative orientation of the polarization of the pump light and the signal light is not known at any given time. Hence, it is important that a Raman amplifier pump source provide non-polarized or depolarized pump light so that the amount of gain is not strongly dependent on the signal polarization.
In the past, different techniques have been used to xe2x80x9cdepolarizexe2x80x9d the light from a polarized light source. One conventional method for converting polarized light to depolarized light is to combine the signals from two polarization maintaining (xe2x80x9cPMxe2x80x9d) optical fibers so that their axes of polarization are at an angle of 90xc2x0 relative to each other. Another method is shown in U.S. Pat. No. 5,692,082 in which polarized light from a laser diode is coupled into a PM fiber such that a plane of polarization of the light is at an angle of 45xc2x0 relative to the polarization axis of the fiber. The length of the PM fiber is set so that an optical path length difference for the two polarization modes is greater than the coherence length of the incident light. Therefore, the two polarization modes are phase decorrelated, and the polarization state of the light output from the fiber is effectively randomized.
As discussed above, known Raman pump sources rely on combining an orthogonally polarized pair of semiconductor laser beams to obtain depolarized pump light. As is understood in the art, in order to maintain depolarized pump light, the power output of the two lasers must be maintained the same, or nearly the same. Depolarized pump light requires that the power from the individual lasers operate at equal levels throughout the operating lifetime of the amplifier. Inaccuracies in monitoring pump powers and changes in the insertion loss of optical components prior to combining the laser beams limit the accuracy with which the power in each polarization can be accurately controlled.
In accordance with the teachings of the present invention, an optical transmission system is disclosed that employs a Raman amplifier including an optical pump for introducing depolarized pump light into the fiber. The pump includes at least one optical source generating a polarized optical pump signal, and an optical splitter that splits the optical pump signal from the optical source into a first pump portion and a second pump portion. Further, the pump includes a delay device that delays the first pump portion relative to the second pump portion, and a beam combiner that combines the delayed first pump portion and the second pump portion to form the depolarized pump light.
In one embodiment, the delay device is a length of fiber that allows the first pump portion to propagate farther from the beam splitter to the beam combiner than the second pump portion. The length of fiber is longer than the coherence length of the pump signal. The pump may also employ two optical sources that apply two polarized pump signals to separate inputs of the beam splitter. In one embodiment, the system further includes at least one erbium doped fiber amplifier.
Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.